Electrolytic Treatment of Azo Dye Wastewaters: Impact of Matrix Chloride Content

Direct Red 83 (10 mM) was electrolytically treated with a graphite anode paired with a stainless steel cathode at 0.05 mA/mm2. Electrolyte composition was varied from 0 to 0.05 N sodium chloride in increments of 0.01 N, using sodium sulfate as a makeup salt to keep the total electrolyte strength at 0.05 N. Fraction of dye remaining was plotted versus time, and first order curves were fitted to the data. Apparent first order rate constants ranged from 3.96×10^-5 to 1.24×10^-3 s^-1. Both direct oxidation by applied current and indirect oxidation by electrolytically formed chemical species were responsible for color removal. The rate of indirect oxidation was slower than the rate of chlorinated oxidizing species production above 0.01 N sodium chloride concentration. Decolorization rate increased with increasing chloride concentration to a rate-limited point occurring at 0.04 N sodium chloride in the system studied.